Change VirtualDisplayAdapter#shouldOnlyMirror()

When flag VIRTUAL_DISPLAY_FLAG_AUTO_MIRROR is set, the display should
start mirroring when there is no content on it, rather than always
mirroring. Therefore, an activity can still be launched on a display
where VIRTUAL_DISPLAY_FLAG_AUTO_MIRROR is set.

This CL also changes the conditions for
ContentRecorder#startRecordingIfNeeded(), because this type of display
can start mirroring once there is no content on it, even if it can
still host tasks.

Also see the JavaDoc of DisplayManager#VIRTUAL_DISPLAY_FLAG_AUTO_MIRROR.

Bug: 424049071
Test: ActivityBlockingTest
Test: VirtualDeviceMirrorDisplayTest
Test: ContentRecorderTests
Flag: EXEMPT bugfix
Change-Id: Ifcc1fa77a5bc128a0a0486822bd3563d8ad52f2e